The present invention relates to a logic circuit, and particularly to a logic circuit employing IIL (integrated injection logic) circuits.
A logic circuit employing IIL circuits has such advantages that it can be highly integrated without requiring isolation regions to individually isolate the elements, and that it can be constructed in the form of a semiconductor integrated circuit device containing analog circuits and digital circuits, without deteriorating the characteristics in the analog circuit portion.
To operate the IIL circuits, however, a constant current called an injection current must be supplied at all times. It was clarified through the study by the inventors of the present invention that if it is attempted to form a large ROM (readonly memory) using IIL circuits, the consumption of electric power, i.e., the sum of constant currents flowing into the individual IIL circuits becomes so great that it is difficult to dissipate the heat that is generated.
It was also found that as the number of IIL circuits increases, the fan out becomes very great to drive logic inputs for the IIL circuits, making it necessary to employ a drive buffer having an extremely large output capacity.
For instance, when a ROM that operates on a logic input of 8-bit strings is to be constituted with IIL circuits each capable of forming 16 collector electrodes (i.e., using IIL circuits each having 16 collectors), it is necessary to supply electric current in an amount to drive 256 IIL circuits simultaneously and to provide a drive capacity to meet the current capacity. In practice, therefore, it is very difficult to realize such a ROM.
The present invention was accomplished by giving attention to the above difficulty, and its object is to provide a logic circuit technique which makes it possible to obtain logic circuits constituted by IIL circuits or logic circuits consisting chiefly of such memory circuits as ROM's of large scale, without so increasing the consumption of electric power and without strikingly increasing the capacity of the drive buffer.